Presently, when an audio/video stream or other content such as pay-TV content or similar are delivered to a user by a provider, the content provider may be willing to watermark this content in a way that is unique for each user. Thus, if the content has been illegally redistributed, the provider may find the source of this illegal distribution.
Usually, a watermark is applied to the content itself, the watermark being formed by an invisible/inaudible modification of the video and/or audio stream. For unicast streams, it is possible to apply the watermark at the head-end or at the reception device. At the head-end, applying a watermark requires modifying the stream for each user. This solution requires a lot of resources and is quite difficult to implement.
If the watermark is applied at the reception, the user units must be secure devices, which is usually not the case if the user unit is a computer type receiver. Moreover, applying the watermark at the reception is a heavy processing.
Thus, there is a need for a method enabling applying a watermark to a content, which is compatible with a unicast stream.
The publication of P. Y. Liew and M. A. Armand, entitled “Inaudible watermarking via phase manipulation of random frequencies”, “MULTIMEDIA TOOLS APPL.” vol. 35, no. 3, 6 Jun. 2007 (2007-06-06), XP019555269, pages 357-377 describes a method for watermarking an audio content in a robust and inaudible way. This method exploits the fact that the human auditory system is not sensitive to absolute phase of a signal. Thus, the human auditory system is unable to discern any audible difference between two signals of the same frequency, but for example 60° out of phase when both are heard separately.
The method described in this document comprises the following steps.
1. Generate, via a pseudorandom generator, 300 random numbers between 0 and 0.5.
2. Multiply each number generated by 44.1×103 to yield a frequency value between 0 and 22.05 kHz.
3. Sort the corresponding 300 frequency components of the audio segment in terms of their amplitudes.
4. Identify 200 frequency values corresponding to the frequency components of smallest amplitude. Then, without changing the order in which they were generated, partition them into two groups of equal cardinality with the first 100 frequency values contained in the first group.
5. To insert watermark bit 0, the phase values of the frequency components of the audio segment corresponding to the first and second groups are set to −2π/3 and 2π/3 rad, respectively. To insert watermark bit 1, the phase values of the frequency components of the audio segment corresponding to the first and second groups have their phase values set to 2π/3 and −2π/3 rad, respectively.
This method differs from the method of the present invention at least by the fact that the method of the present invention does not rely on several signal being out of phase.
The document above contains a short description of several methods enabling adding a watermark to an audio content. According to one of those methods, referred to as “Time base modulation”, the audio signal is first subdivided into segments. Each segment is then expanded or compressed depending on the value of a watermark bit for this segment.
To retrieve the watermark, the watermarked sequence is compared to the original to detect the deviations resulting from the various sequences of expansions and compressions.
This method requires the comparison between the original signal and the watermarked signal in order to retrieve the watermark as, whatever the watermark is, the quantity of information within one packet remains unchanged. Moreover, the watermark may be audible if the expansions or compressions changes are too great.
The publication entitled “Length based network steganography using UDP protocol” from Anand S Nair, Abhishek Kumar, Arijit Sur and Sukumar Nandi, 3RD INTERNATIONAL CONFERENCE ON IEEE, 27 May 2011, pages 726-730, XP032050871, ISBN: 978-1-61284-485-5 discloses a steganographic algorithm for hiding secret data within packets of content. The length of the packets is modulated in order to correspond to the secret data to be transmitted. The algorithm described in this document enables imitating the length distribution of conventional packets, i.e. packets without steganographic data, in order to prevent users from guessing that secret data is added to the data flow.
In this document, the key is the use of steganography. This means that data is hidden within the packets. If an ill-intentioned person learns that secret data is hidden within the data, this data can be easily eliminated and even possibly read. Moreover, the secret data is very sensitive to re-sampling.